Ferroelectric capacitors are of interest today because of their use in ferroelectric memories and thin film discrete capacitors. See U.S. Pat. Nos. 4,873,664; 4,809,225; 4,853,893; 4,918,654; 4,910,708; 4,914,627; 4,893,272; 4,888,733, all issued to Ramtron Corporation of Colorado Springs, Colorado, USA.
Platinum (Pt) electrodes are often used in the fabrication of ferroelectric capacitors. Unfortunately, currently a number of problems exist with the use of these Pt electrodes in ferroelectric capacitors. One advance in dealing with this problem is disclosed in EPO publication 404,295 (Al) which is based on U.S. Ser. No. 368,668, now U.S. Pat. No. 5,005,102 ("Multilayer Electrodes For Integrated Circuit Capacitors").
Ferroelectric capacitors are fabricated using an oxide known as "PZT" which refers generally to a mixture or solution, comprised of oxides of lead, zirconium and titanium, forming a perovskite structure. The PZT is sandwiched between electrodes typically comprised of noble metals, terminal lower and upper electrodes. The lower electrode is deposited directly on silicon dioxide or some other suitable dielectric. When using Pt (or another noble metal) in the fabrication of a ferroelectric capacitor the adhesion at the interface between the Pt or other noble metals and the dielectric is poor. If there is poor adhesion at this interface, delamination can potentially occur between the electrode and the dielectric.
Presently, one approach for solving this adhesion problem is to use titanium (Ti) or chromium (Cr) under the Pt as a "glue" layer. However, both Ti and Cr can readily diffuse through the Pt. If the Ti or Cr does diffuse through the Pt and is able to reach the PZT interface, the Ti or Cr will oxidize and will result in the degradation of the electrical properties of the capacitor.
When using conventional physical deposition techniques, Pt typically tends to orient with the (111) crystal planes of Pt parallel to most substrates. Unfortunately, when using these techniques, the orientation of Pt cannot be changed or controlled. Therefore, one is limited to an orientation parallel to the substrate.
In addition to Pt, other materials have been suggested for use as electrodes in fabricating ferroelectric capacitors. For example, indium tin oxide (ITO) is commonly mentioned as a possible electrode in the literature. However, the conductivity of ITO is too low for use in integrated circuits. Further, the possibility of epitaxy with ITO is not mentioned in the literature. Aluminum is another metal which has been used as the top electrode in a ferroelectric capacitor. See Ramtron's EPO 404,295 (Al), supra. Unfortunately, aluminum oxide can form at the PZT to aluminum interface and cause a significant degradation of the electrical properties of thin (less than one micron) films.
Accordingly, the object of the present invention is to create a ferroelectric capacitor structure that overcomes the deficiencies of the prior devices.